CENTRIFUGAL PUMPS - Modern Design and Practices

Softfoot                   

The term Best Efficiency Point, or BEP, refers to the pump flow at a specified impeller diameter where efficiency is highest on its performance curve. The BEP may be based on rated impeller diameter, such as per by API 610 definition, or on maximum diameter such as in calculating specific speed (Ns) and suction specific speed (Nss).

The flow at BEP is optimal for a given set of pump hydraulics. It is the point on the curve where the hydraulic losses are minimal – that point is independent of the pump mechanical design. In evaluating BEP, the flow (e.g. 1000 GPM) is of primary interest rather than the efficiency itself (e.g. 85%), unless the BEP is used as parameter to find the Ns for the pump selection that will yield the highest possible rated efficiency. 

The location of BEP on a curve is the result of the hydraulic match of the impeller and volute (or diffuser, in a diffuser type pump); it may, or many not, coincides with the impeller design flow.  An impeller will give different values of BEP if used in casings with different volutes. Similarly, a volute will also give different BEP if used with different impellers. A pump BEP is normally lower than the impeller shockless entrance flow (Qse) because of suction losses.

A pump whose BEP coincides with the impeller design flow means its impeller is matched to the volute optimally – the volute throat area is correctly sized for the flow. An undersized throat area will pull the BEP to the left, whereas an oversized area will push the BEP to the right, of the pump curve. There are theoretical and empirical methods that can be used to estimate the shift in the location of BEP on a pump curve based on changes in the volute throat areas.

BEP applications

BEP is widely used as reference point in defining a pump hydraulic design and in setting limits to its operation. Examples: 

a. Parameters such as specific speed (Ns), suction specific speed (Nss), and hydrodynamic size (Z) are calculated from BEP at maximum impeller diameter. If the curve were plotted at a cut diameter, the flow and head should be extrapolated to its maximum diameter to calculate Ns, Nss, and Z using Affinity Laws. 

b. It is common practice to set the end-of-curve (EOC) as a percentage of BEP - typically at 125%. This is to prevent oversizing the driver HP and to avoid excessive flow velocity if the pump were operating at run-out flow.

c. For viscous service, the pump performance curve viscosity correction factors for flow, head, and efficiency are based on BEP. The previous practice was to use the BEP at maximum impeller diameter but current practice allows the use of BEP at rated diameter.

d. It is common practice to set a pump's recommended minimum continuous stable flow (MCSF), allowable operating range (AOR), and preferred operating range (POR) as some percentages of BEP.

e. The empirical radial thrust factor, K, used in thrust calculations is based on the relative position of the rated flow to BEP. The hydraulic thrust load and shaft deflection are lowest when a pump operates at, or close to, BEP.

f. Many users prefer that their pumps operate within 80% to 110% of BEP for optimum performance, to minimize hydraulic radial and axial thrust loads, and to avoid vibration issues induced by low flow recirculation. 

g. The pump noise level (in dBA) is affected by the rated flow’s relative position to its BEP – the farther away the rated flow is from BEP, the higher is the noise level. 

h. Many specifications require the rated and maximum flows to straddle the BEP, thus the location of BEP becomes a factor in the pump curve selection.

Risks of non-compliance with BEP-related specs

Although the use of BEP is widely popular there is still a disconnect between understanding its theory and practical use, particularly if it involved a pass/fail pump test scenarios. Here are two examples:

Example 1: It was specified that a pump rated flow of 1000 GPM shall not be less than 80% of its BEP. The pump was initially rejected because its BEP, as tested, was at 1280 GPM, thus making its rated flow at 78% of BEP.

Solution: Firstly, it was considered to trim the impeller diameter to pull back the BEP to below 1250 GPM while keeping the head within the specified -3% tolerance. This solution was rejected as being too risky because there was no guarantee that the target BEP and head tolerance can be both achieved. The eventual solution was to modify the volute and reduce its throat area to pull back the BEP well below 1250 GPM without risking the head being out of tolerance.

Example 2: A pump specification required that the pump suction specific speed (Nss) shall not exceed 11,000. The pump test showed the Nss slightly exceeding the limit. The customer rejected the pump as tested and asked that it be modified to comply with the Nss limit.

Solution: Knowing that the NPSHR curve is non-linear but is somewhat quadratic or parabolic in shape it was decided to cut back the volute lips to increase its throat area and move the BEP to the right. Based on the new BEP position and the quadratic rise in NPSHR, the Nss was reduced below 11,000 and the pump was accepted.

These examples shows how the concept of BEP has been misused to the point that it becomes a number’s game. The effort, cost, and time spent to adjust the position of BEP to make the pumps compliant with the specs were simply wasted because it did not make the pumps any better.

The uncertainty of a pass/fail scenario of BEP-related specs can be avoided if the pump industry adopted a BEP tolerance similar manner to the widely accepted performance tolerance on flow, head, and efficiency. This author is a strong proponent of a BEP tolerance and is suggesting a tolerance of + or – 2%. Under such a standard, any flow within the 4% spread of the tested BEP should be acceptable and could be used in calculating certain BEP-based parameters.

Factors that may cause variations in BEP

Indeed, it will be unfortunate to accept or reject a pump simply on the basis of its BEP that will throw off some BEP-related parameters that would be rendered non-compliant. Identical pumps, even those manufactured and tested on the same job lot, may vary in BEP because they are not hard numbers but are just very close approximations. Some factors that may cause the BEP to deviate include:

1. Many curve-plotting programs use the best-fit algorithm where minor changes in some data points will cause a shift in the location of BEP. Even if the same set of data points are used in plotting the curve, a shift in BEP may occur if the degree of fit is changed in the algorithm.

2. Differences in pattern equipment – foam pattern, wood pattern, investment pattern, etc. have different pattern set-up and shrinkages that can result in dimensional variations in castings. Variations in impeller width “BA” and volute throat area (Av) will have significant effect on the location of BEP.

3. Differences in impeller and casing material will result in variations in casting shrinkage and surface finish. These may cause a shift in the pump curve BEP.

4. Variations in machining tolerance, clearances, fits, and assembly.

5. Differences in test procedure, test set-up, and calibration of test instruments.

The need for BEP tolerance

A BEP is not a hard number that is cut in stone for any pump. The location of BEP in a pump curve can change easily due to factors mentioned above. Relying on BEP to set acceptable parameters such as Ns, Nss, MCSF, AOR, or POR, etc. can be quite risky and uncertain.

To reduce this uncertainty, especially if it involved a test pass/fail situation, the pump industry must set a standard for BEP tolerance. If tolerances in flow, head, and efficiencies are the norm, why not a standard tolerance on BEP? This will remove the uncertainties or disagreements in what would be an acceptable pump performance test.

This author is a strong proponent of a BEP tolerance and is suggesting a + / – 2% tolerance where any flow within this BEP flow range can be considered as the BEP in doing some pump parameter calculations.

For example, it is still a common practice to specify that pumps with Nss of 11,000 shall not be acceptable. In instances where compliance with this spec is borderline, a small shift in BEP to the right could mean a “pass” whereas a shift to the left could mean a “fail” for the performance test results.